fix form submission on edit event page w/ multiple dates.
fix form submission on edit event page w/ multiple dates.

fixes CNVS-3616

when creating/editing an event with different dates for
each section, properly submit the form w/o throwing a
javascript error.

test plan:
  * create an event for a course with multiple sections,
    and check its 'use a different date for each section'
    checkbox;
  * provide a date for each section, click 'update event,'
    and verify that the form is submitted and the event is
    updated properly.

Change-Id: I336ecf5be45a27aec8f2c6df5a509272b4d19393
Reviewed-on: https://gerrit.instructure.com/17443
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Joel Hough <4816dabf8db1bc6cac35b3a24cab2ff844b5b0c7@instructure.com>
QA-Review: Marc LeGendre <d33c80bc45d65303e33ca83108a9952b745af9ef@instructure.com>

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